1. Field of the Invention
This invention relates generally to coated articles and, in particular, to multi-layer coated articles having a functional topcoat and at least one undercoating layer.
2. Technical Considerations
Articles having multi-layer coatings are used in a wide variety of applications. One example is in the field of thin film solar cells. A typical solar cell comprises a substrate, such as a glass plate, having a transparent conductive film (first electrode). A semiconductor film having a photoelectric conversion material is deposited on the transparent conductive film. The cell includes another substrate having a transparent conductive film (second electrode). An electrolyte could be enclosed between the two electrodes. When the photoelectric conversion material adsorbed on the semiconductor film is irradiated, electrons generated by the irradiation move through the semiconductor film and into one of the transparent conductive films. For example, the electrons can move through the first electrode, through an electrical lead, and to the other electrode. For solar cells, it is important for photoelectric conversion efficiency that the electrons move as rapidly as possible through the first conductive film to the other electrode. That is, it is desirable if the surface resistivity of the transparent conductive film is low. If the electrons do not move rapidly, recombination of the electrons with the photoelectric conversion material (conventionally referred to as “reverse current” or “back current”) can occur. It is also desirable if the conductive film is highly transparent to permit the maximum amount of solar radiation to pass to the photoelectric conversion material. Therefore, it would be desirable to provide a coated article for a solar cell that enhances the electron flow through a transparent conductive film. That is, a transparent conductive film having a low surface resistivity.
Another example of a field utilizing coated articles is the field of photocatalytic articles. It is known to apply a photocatalytic coating, such as titania, onto a substrate to provide a coated article having self-cleaning properties. Upon exposure to certain electromagnetic radiation, such as ultraviolet radiation, the photocatalytic coating interacts with organic contaminants on the coating surface to degrade or decompose the organic contaminants. However, conventional photocatalytic articles have a relatively high visible light reflectance and, therefore, can be inappropriate for use in some architectural applications. Additionally, conventional photocatalytic coatings can be subject to degradation through what is conventionally termed “sodium ion poisoning” caused by sodium ions defusing from the underlying glass substrate into the photocatalytic coating. Further, conventional photocatalytic coatings tend to display iridescence effects that detract from the aesthetic appearance of the coated article.
Therefore, it would be desirable to provide a coated article having an undercoating layer positioned between a substrate and a functional top coat (such as but not limited to a conductive photovoltaic transparent conductive coating or a photocatalytic coating) that not only acts as a barrier to sodium ion diffusion but also enhances the performance of the coated article. For example, the performance could be enhanced by decreasing the reflectance of the coated article and/or providing color suppression to the article and/or increasing the functionality of the top coat. For example, in photovoltaic applications, the undercoating layer could decrease the surface resistivity of the top coat (e.g., a transparent conductive layer) to increase electron flow. In photocatalytic applications, the undercoating layer could increase the photocatalytic activity of the photocatalytic coating.